27 research outputs found
Self-induced Scattering of Strahl Electrons in the Solar Wind
We investigate the scattering of strahl electrons by microinstabilities as a
mechanism for creating the electron halo in the solar wind. We develop a
mathematical framework for the description of electron-driven
microinstabilities and discuss the associated physical mechanisms. We find that
an instability of the oblique fast-magnetosonic/whistler (FM/W) mode is the
best candidate for a microinstability that scatters strahl electrons into the
halo. We derive approximate analytic expressions for the FM/W instability
threshold in two different regimes, where is the ratio of the core electrons' thermal pressure to the magnetic
pressure, and confirm the accuracy of these thresholds through comparison with
numerical solutions to the hot-plasma dispersion relation. We find that the
strahl-driven oblique FM/W instability creates copious FM/W waves under
low- conditions when ,
where is the strahl speed and is the thermal
speed of the core electrons. These waves have a frequency of about half the
local electron gyrofrequency. We also derive an analytic expression for the
oblique FM/W instability for . The comparison of our
theoretical results with data from the \emph{Wind} spacecraft confirms the
relevance of the oblique FM/W instability for the solar wind. The whistler
heat-flux, ion-acoustic heat-flux, kinetic-Alfv\'en-wave heat-flux, and
electrostatic electron-beam instabilities cannot fulfill the requirements for
self-induced scattering of strahl electrons into the halo. We make predictions
for the electron strahl close to the Sun, which will be tested by measurements
from \emph{Parker Solar Probe} and \emph{Solar Orbiter}.Comment: 11 pages, 11 figure
Tracking a beam of electrons from the low solar corona into interplanetary space with the Low Frequency Array, Parker Solar Probe and 1 au spacecraft
Type III radio bursts are the result of plasma emission from mildly
relativistic electron beams propagating from the low solar corona into the
heliosphere where they can eventually be detected in situ if they align with
the location of a heliospheric spacecraft. Here we observe a type III radio
burst from 0.1-16 MHz using the Parker Solar Probe (PSP) FIELDS Radio Frequency
Spectrometer (RFS), and from 10-80 MHz using the Low Frequency Array (LOFAR).
This event was not associated with any detectable flare activity but was part
of an ongoing noise storm that occurred during PSP encounter 2. A deprojection
of the LOFAR radio sources into 3D space shows that the type III radio burst
sources were located on open magnetic field from 1.6-3 and originated
from a specific active region near the East limb. Combining PSP/RFS
observations with WIND/WAVES and Solar Terrestrial Relations Observatory
(STEREO)/WAVES, we reconstruct the type III radio source trajectory in the
heliosphere interior to PSP's position, assuming ecliptic confinement. An
energetic electron enhancement is subsequently detected in situ at the STEREO-A
spacecraft at compatible times although the onset and duration suggests the
individual burst contributes a subset of the enhancement. This work shows
relatively small-scale flux emergence in the corona can cause the injection of
electron beams from the low corona into the heliosphere, without needing a
strong solar flare. The complementary nature of combined ground and space-based
radio observations, especially in the era of PSP, is also clearly highlighted
by this study.Comment: 17 pages, 10 figures, Submitted to ApJ, April 15 202
The statistical properties of solar wind temperature parameters near 1 AU
We present a long-duration (10 years) statistical analysis of the
temperatures, plasma betas, and temperature ratios for the electron, proton,
and alpha-particle populations observed by the \emph{Wind} spacecraft near 1
AU. The mean(median) scalar temperatures are
12.2(11.9) eV, 12.7(8.6) eV, and
23.9(10.8) eV. The mean(median) total
plasma betas are 2.31(1.09),
1.79(1.05), and 0.17(0.05). The mean(median) temperature ratios are
1.64(1.27),
1.24(0.82), and
2.50(1.94). We also examined these parameters during time intervals that
exclude interplanetary (IP) shocks, times within the magnetic obstacles (MOs)
of interplanetary coronal mass ejections (ICMEs), and times that exclude MOs.
The only times that show significant alterations to any of the parameters
examined are those during MOs. In fact, the only parameter that does not show a
significant change during MOs is the electron temperature. Although each
parameter shows a broad range of values, the vast majority are near the median.
We also compute particle-particle collision rates and compare to effective
wave-particle collision rates. We find that, for reasonable assumptions of wave
amplitude and occurrence rates, the effect of wave-particle interactions on the
plasma is equal to or greater than the effect of Coulomb collisions. Thus,
wave-particle interactions should not be neglected when modeling the solar
wind.Comment: 23 pages, 3 figures, 6 tables, submitted to Astrophys. J. Suppl. on
Jan. 30, 201
Electron Energy Partition across Interplanetary Shocks. III. Analysis
An analysis of model fit results of 15,210 electron velocity distribution functions (VDFs), observed within 2 hr of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 au, is presented as the third and final part on electron VDFs near IP shocks. The core electrons and protons dominate in the magnitude and change in the partial-to-total thermal pressure ratio, with the core electrons often gaining as much or more than the protons. Only a moderate positive correlation is observed between the electron temperature and the kinetic energy change across the shock, while weaker, if any, correlations were found with any other macroscopic shock parameter. No VDF parameter correlated with the shock normal angle. The electron VDF evolves from a narrowly peaked core with flaring suprathermal tails in the upstream to either a slightly hotter core with steeper tails or much hotter flattop core with even steeper tails downstream of the weaker and strongest shocks, respectively. Both quasi-static and fluctuating fields are examined as possible mechanisms modifying the VDF, but neither is sufficient alone. For instance, flattop VDFs can be generated by nonlinear ion acoustic wave stochastic acceleration (i.e., inelastic collisions), while other work suggested they result from the combination of quasi-static and fluctuating fields. This three-part study shows that not only are these systems not thermodynamic in nature; even kinetic models may require modification to include things like inelastic collision operators to properly model electron VDF evolution across shocks or in the solar wind.Peer reviewe
Electron Energy Partition across Interplanetary Shocks. II. Statistics
A statistical analysis of 15,210 electron velocity distribution function (VDF) fits, observed within +/- 2 hr of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 au, is presented. This is the second in a three-part series on electron VDFs near IP shocks. The electron velocity moment statistics for the dense, low-energy core, tenuous, hot halo, and field-aligned beam/strahl are a statistically significant list of values illustrated with both histograms and tabular lists for reference and baselines in future work. Given the large statistics in this investigation, the beam/strahl fit results in the upstream are now the most comprehensive attempt to parameterize the beam/strahl electron velocity moments in the ambient solar wind. The median density, temperature, beta, and temperature anisotropy values for the core(halo)[beam/strahl] components, with subscripts ec(eh)[eb], of all fit results, respectively, are n(ec(h)[b]) similar to 11.3(0.36)[0.17] cm(-3), T-ec(h)[b],T-tot similar to 14.6(48.4)[40.2] eV, beta(ec(h)[b],tot) similar to 0.93(0.11)[0.05], and Alpha(ec(h)[b]) similar to 0.98(1.03)[0.93]. This work will also serve as a 1 au baseline and reference for missions like Parker Solar Probe and Solar Orbiter.Peer reviewe
Electron Energy Partition across Interplanetary Shocks. I. Methodology and Data Product
Analyses of 15,314 electron velocity distribution functions (VDFs) within +/- 2 hr of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 au are introduced. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar VDF, while both the halo and beam/strahl components were best fit to bi-kappa VDF. This is the first statistical study to show that the core electron distribution is better fit to a self-similar VDF than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The ranges of values defined by the lower and upper quartiles for the kappa exponents are k(ec) similar to 5.40-10.2 for the core, k(eh) similar to 3.58-5.34 for the halo, and k(eb) similar to 3.40-5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents is s(ec) similar to 2.00-2.04, and those of asymmetric bi-self-similar core exponents are p(ec) similar to 2.20-4.00 for the parallel exponent and q(ec) similar to 2.00-2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study.Peer reviewe
Parker Solar Probe Observations of High Plasma Beta Solar Wind from Streamer Belt
In general, slow solar wind from the streamer belt forms a high plasma beta
equatorial plasma sheet around the heliospheric current sheet (HCS) crossing,
namely the heliospheric plasma sheet (HPS). Current Parker Solar Probe (PSP)
observations show that the HCS crossings near the Sun could be full or partial
current sheet crossing (PCS), and they share some common features but also have
different properties. In this work, using the PSP observations from encounters
4 to 10, we identify streamer belt solar wind from enhancements in plasma beta,
and we further use electron pitch angle distributions to separate it into HPS
solar wind that around the full HCS crossings and PCS solar wind that in the
vicinity of PCS crossings. Based on our analysis, we find that the PCS solar
wind has different characteristics as compared with HPS solar wind: a) PCS
solar wind could be non-pressure-balanced structures rather than magnetic
holes, and the total pressure enhancement mainly results from the less reduced
magnetic pressure; b) some of the PCS solar wind are mirror unstable; c) PCS
solar wind is dominated by very low helium abundance but varied alpha-proton
differential speed. We suggest the PCS solar wind could originate from coronal
loops deep inside the streamer belt, and it is pristine solar wind that still
actively interacts with ambient solar wind, thus it is valuable for further
investigations on the heating and acceleration of slow solar wind
The Structure and Origin of Switchbacks: Parker Solar Probe Observations
Switchbacks are rapid magnetic field reversals that last from seconds to
hours. Current Parker Solar Probe (PSP) observations pose many open questions
in regards to the nature of switchbacks. For example, are they stable as they
propagate through the inner heliosphere, and how are they formed? In this work,
we aim to investigate the structure and origin of switchbacks. In order to
study the stability of switchbacks, we suppose the small scale current sheets
therein may work to braid and stabilize the switchbacks. Thus, we use the
partial variance of increments method to identify the small scale current
sheets, and then compare their distributions in switchbacks. With more than one
thousand switchbacks identified with PSP observations in seven encounters, we
find many more current sheets inside than outside switchbacks, indicating that
these micro-structures should work to stabilize the S-shape structures of
switchbacks. Additionally, with the helium measurements, we study the
variations of helium abundance ratios and alpha-proton differential speeds to
trace switchbacks to their origins. We find both helium-rich and helium-poor
populations in switchbacks, implying the switchbacks could originate from both
closed and open magnetic field regions in the Sun. Moreover, we observe that
the alpha-proton differential speeds also show complex variations as compared
to the local Alfv\'en speed. The joint distributions of both parameters show
that low helium abundance together with low differential speed is the dominant
state in switchbacks. The presence of small scale current sheets in switchbacks
along with the helium features are in line with the hypothesis that switchbacks
could originate from the Sun via interchange reconnection process. However,
other formation mechanisms are not excluded